The objective of this research is to study the biochemical and physiological changes in selenium deficient subhuman primates. Twelve pregnant Rhesus monkeys are planned for this study. Four of these will be fed a basal diet containing 0.2 ppm selenium through-out the entire experiment. The remaining eight will be fed the selenium-deficient basal diet. When the young are born, half of these will be switched to the diet containing 0.2 ppm selenium and the other half continued on this same diet. Blood will be taken at monthly intervals from both the adult and young monkeys for selenium and vitamin E analysis, and for enzyme assays. A complete blood count, and measurements of glutathione peroxidase activity (both plasma and erythrocytes), plasma glutamic-oxalacetic transaminase, lactic dehydrogenase, creatine phosphokinase, and ornithine carbamyl transferase activities are planned on each sample of blood. When the young selenium deficient monkeys begin to become listless, animals from all groups will be killed and a complete necropsy and histopathological study performed. Glutathione peroxidase (both selenium and non-selenium dependent) activities and selenium content will be determined on their tissues. After the young are weaned, selenium balance studies with the adult monkeys are planned with various plant and animal products to obtain information on selenium bioavailability in various food sources for man.